1. Field of the Invention
The present invention relates to a receiver capable of receiving an analog broadcast and a digital broadcast and an integrated circuit (IC) for the same.
2. Description of Related Art
In the current amplitude modulation (AM) broadcast and the current frequency modulation (FM) broadcast, the processing of materials to be broadcast is being digitized. However, the broadcasts themselves are performed in an analog system.
However, if the broadcasts themselves are digitized, a broadcast in high sound quality can be supplied, and additional data service and the like can be enabled, and further the efficiency of frequency utilization can be improved. Accordingly, a digital broadcast, in which broadcast themselves are digitized, is now under consideration.
However, it is a matter of course that a digital broadcast receiver is newly required in place of a conventional analog broadcast receiver for receiving the digital broadcast. On the other hand, it is desirable to change over from the analog broadcast to the digital broadcast smoothly without any resistance. Consequently, the coexistence of the former analog broadcast and the new digital broadcast is required during the period of the changeover.
There is the IBOC (In-Band On Channel) system as a broadcast system enabling the coexistence. The IBOC system realizes the coexistence by employing a frequency disposition of the analog broadcast and the digital broadcast as shown in FIG. 4A, for example.
That is, in FIG. 4A, a reference code SA indicates a broadcast wave signal of a conventional analog FM broadcast, or an FM signal. The carrier frequency of the broadcast wave signal SA is a frequency fRX. In a case of making a digital broadcast wave signal coexist with the analog broadcast wave signal SA, two digital broadcast wave signals SD are disposed adjacently to the both side frequency bands of the broadcast wave signal SA.
However, there is a case where only one of the two digital broadcast wave signals SD is used. In addition, the levels of the digital broadcast wave signals SD are set to be lower than that of the analog broadcast wave signal SA by about 15 dB-20 dB. The program contents of the digital broadcast wave signals SD are generally the same as those of the analog broadcast wave signal SA.
In future, the analog broadcast wave signal SA will be stopped from being broadcast, and only the digital broadcast wave signal SD will be broadcast. Or, the frequency band of the analog broadcast wave signal SA will be used by the digital broadcast. Incidentally, it is regarded that it will take from ten years to fifteen years to change over from the analog broadcast to the digital broadcast.
For coping with the aforesaid coexistence of the analog broadcast and the digital broadcast, a receiver that can be used during the changeover period and after it, namely, a hybrid receiver that can receive both of the analog broadcast and the digital broadcast, is required.
The hybrid receiver can be constructed as shown in FIG. 5, for example. For simplifying the situation, it is supposed that the analog broadcast wave signal SA and both the digital broadcast wave signals SD are broadcast as shown in FIG. 4A. The antenna 1 receives these signals SA and SD. Then, the received signals SA and SD are supplied to a mixer circuit 5 though a signal line composed of an antenna tuning circuit 2, a high frequency amplifier 3 and an interstage tuning circuit 4 in the order. On the other hand, a local oscillation circuit 6 forms a local oscillation signal SLO having a predetermined frequency, and the formed local oscillation signal SLO is also supplied to the mixer circuit 5.
Thus, the frequency conversion of the received signals SA and SD are performed in the mixer circuit 5 by means of the local oscillation signal SLO. As a result of the frequency conversion, as shown in FIG. 4B, the received signals SA and SD are converted to intermediate-frequency signals SAI and SDI in which the carrier frequency fRX of the signal SA becomes a predetermined intermediate frequency fIF. The converted intermediate-frequency signals SAI and SDI are then supplied to a band-pass filter 7. The band-pass filter 7 works as an intermediate-frequency filter and has a wide frequency band passing characteristic as shown by a dashed line 7F in FIG. 4B. Then an output of the band-pass filter 7 is supplied to a band-pass filter 9A and a band-elimination filter 9D through an AGC amplifier 8.
In this case, the band-pass filter 9A is used for eliminating an adjacent disturbance wave signal, and has a center frequency of a frequency fIF and a pass band corresponding to the intermediate-frequency signal SAI, as shown by a dashed line 9AF in FIG. 4C. Consequently, the intermediate-frequency signal SAI Of the analog broadcast is taken out of the band-pass filter 9A.
Besides, the band-elimination filter 9D is used for attenuating the analog intermediate-frequency signal SAI, and has a center frequency of a frequency fIF and an elimination band same as the frequency band of the intermediate-frequency signal SAI, as shown by dashed lines 9DF in FIG. 4D. Consequently, the intermediate-frequency signals SDI of the digital broadcast are taken out of the band-elimination filter 9D.
Accordingly, when only the analog broadcast is broadcast or when both of the analog broadcast and the digital broadcast are broadcast, the analog broadcast can be heard by demodulating the intermediate-frequency signal SAI outputted from the band-pass filter 9A.
On the other hand, when only the digital broadcast is broadcast or when both of the analog broadcast and the digital broadcast are broadcast, the digital broadcast can be heard by decoding the intermediate-frequency signals SDI outputted from the band-elimination filter 9D.
Consequently, any one of the analog broadcast and the digital broadcast in conformity with the IBOC system can be heard by means of the receiver shown in FIG. 5. Furthermore, the receiver can be used as it is even if only the digital broadcast is broadcast in future.
However, the pass band width of the band-pass filter 7 should be wide for taking out the two kinds of the intermediate-frequency signals SAI and SDI. On the other hand, the band-pass filter 7 should eliminate disturbance wave signals from adjacent channels. As a result, the ratio band, i.e. a ratio of a cutoff frequency band to a pass frequency band, becomes small. Consequently, the requirements for the band-pass filter 7 become strict. As a result, the order and/or the scale of the band-pass filter 7 becomes large.
Furthermore, because the band-pass filter 9A and the band-elimination filter 9D are required in addition to the band-pass filter 7, many filters become necessary. Consequently, the increase of consumption current owing to the increase of the circuit scale, the increase of the size of a receiver, the increase of costs, and the like are incurred.
The present invention aims to solve the aforesaid problems. That is, the present invention aims to provide a receiver and an IC that can receive the analog broadcast and the digital broadcast, and that can make the ratio of the band-pass filter for taking out an object intermediate-frequency signal not to be small for preventing the order and scale of the bandpass filter from increasing, and further that can prevent the increase of the circuit scale, the increase of the size of a receiver, the increase of costs, and the like.
According to a first aspect of the present invention, there is provided a receiver for a broadcast system in which a broadcast wave signal of an analog broadcast and a broadcast wave signal of a digital broadcast are transmitted in frequency bands adjacent to each other, comprising: mixer means, to which a received signal and a local oscillation signal are supplied, and which performs a frequency conversion of the received signal to an intermediate-frequency signal by means of the local oscillation signal; filter means which takes the intermediate-frequency signal out of an output signal of the mixer means; and demodulation means which takes a signal indicating broadcast contents out of the intermediate-frequency signal taken out by means of the filter means, wherein: at a time of receiving the analog broadcast, a frequency of the local oscillation signal is made to be a first frequency within a frequency band of the digital broadcast adjacent to a frequency band of the analog broadcast received by the receiver, and the signal taken out by said modulating means is a signal indicating the contents of the analog broadcast, and, at a time of receiving the digital broadcast, the frequency of the local oscillation signal is made to be a second frequency equal to a carrier frequency of the analog broadcast adjacent to the frequency band of the digital broadcast received by the receiver, and the signal taken out by said modulating means is a signal indicating the contents of the digital broadcast.
According to a second aspect of the present invention, there is provided a receiver for a broadcast system in which a broadcast wave signal of an analog broadcast and a broadcast wave signal of a digital broadcast are transmitted in frequency bands adjacent to each other, comprising: first and second mixer means, to each of which a received signal and a local oscillation signal are supplied, and each of which performs a frequency conversion of the received signal to first and second intermediate-frequency signals having phases orthogonal to each other by means of the first local oscillation signal; first and second filter means each of which takes the first and second intermediate-frequency signals out of output signals of the first and second mixer means, respectively; demodulation means which takes a signal indicating broadcast contents out of the first and second intermediate-frequency signals taken out by means of the first and second filter means, wherein: at a time of receiving the analog broadcast, a frequency of the local oscillation signal is made to be a first frequency within a frequency band of the digital broadcast adjacent to a frequency band of the analog broadcast received by the receiver, and the signal taken out by said demodulation means is a signal indicating the contents of the analog broadcast, and, at a time of receiving the digital broadcast, the frequency of the local oscillation signal is made to be a second frequency equal to a carrier frequency of the analog broadcast adjacent to the frequency band of the digital broadcast received by the receiver, and the signal taken out by said demodulation means is a signal indicating the contents of the digital broadcast.
According to a third aspect of the present invention, there is provided an IC for a broadcast system in which a broadcast wave signal of an analog broadcast and a broadcast wave signal of a digital broadcast are transmitted in frequency bands adjacent to each other, comprising: mixer means, to which a received signal and a local oscillation signal are supplied, and which performs a frequency conversion of the received signal to an intermediate-frequency signal by means of the local oscillation signal; and filter means which takes the intermediate-frequency signal out of an output signal of the mixer means, wherein: at a time of receiving the analog broadcast, a frequency of the local oscillation signal is made to be a first frequency within a frequency band of the digital broadcast adjacent to a frequency band of the analog broadcast received by the receiver, at a time of receiving the digital broadcast, the frequency of the local oscillation signal is made to be a second frequency equal to a carrier frequency of the analog broadcast adjacent to the frequency band of the digital broadcast received by the receiver, and a signal indicating broadcast contents of one of the analog broadcast and the digital broadcast is taken out of the intermediate-frequency signal taken out by means of the filter means.
According to a fourth aspect of the present invention, there is provided an IC for a broadcast system in which a broadcast wave signal of an analog broadcast and a broadcast wave signal of a digital broadcast are transmitted in frequency bands adjacent to each other, comprising: first and second mixer means, to each of which a received signal and a local oscillation signal are supplied, and each of which performs a frequency conversion of the received signal to a first and second intermediate-frequency signals having phases orthogonal to each other by means of the first local oscillation signal; and first and second filter means each of which takes the first and second intermediate-frequency signals out of output signals of the first and second mixer means, respectively, wherein: at a time of receiving the analog broadcast, a frequency of the first local oscillation signal is made to be a first frequency within a frequency band of the digital broadcast adjacent to a frequency band of the analog broadcast received by the receiver and, at a time of receiving the digital broadcast, the frequency of the first local oscillation signal is made to be a second frequency equal to a carrier frequency of the analog broadcast adjacent to the frequency band of the digital broadcast received by the receiver, and a signal indicating broadcast contents of the one of analog broadcast and the digital broadcast is taken out of the first and second intermediate-frequency signals taken out by means of the first and second filter means.
Accordingly, the analog broadcast is received through the superheterodyne system and the digital broadcast is received through the direct conversion system.
According to the aforesaid aspects of the present invention, an image rejection characteristic and an adjacent disturbance rejection characteristic necessary for receiving an analog broadcast or a digital broadcast can be obtained. In addition, the intermediate frequency can be lowered. Consequently, the receiving circuit including an intermediate-frequency filter can be made to be an IC. Moreover, almost all of the circuits can be shared in a case of installing them in an IC, and thereby it can be realized to miniaturize the shape thereof and to lower the price thereof.
Furthermore, the frequency bandwidths to be dealt with in a main signal line are the same at the time of receiving the digital broadcast and at the time of receiving the analog broadcast. Consequently, it becomes easy to process signals. Besides, the ratio band of the band-pass filter for taking out an object intermediate-frequency signal does not become small, and then the order and scale of the band-pass filter are realized to be small.
Furthermore, even if the analog broadcast wave signal is stopped and the frequency band of the analog broadcast wave signal is used for the digital broadcast in future, the present invention can cope with such a case as a receiver and an IC exclusive to the digital broadcast only by modifying the passing band of the band-pass filter.